1. Field of the Invention
The present invention relates to an apparatus for using a duplex transmission line in a network.
2. Description of the Related Art
In general, a ring-type local area network comprises, as shown in FIG. 1A, a plurality of node stations (STs) 30, and a duplex transmission line 31 for connecting the STs 30 in a ring shape. Each ST 30 comprises a transmission line interface section 32, which includes a receiving section, a transmitting section, a line changing section and a line control section (these not shown), and a serial-parallel converter (SPC) 33 for performing serial/parallel conversion of data transmitted from/to the line changing section of the transmission line interface section 32. A device 36 such as a computer or a terminal is connected to the SPC 33 through an internal bus 34 and an ST interface (STI) 35.
The duplex transmission line 31 comprises an operating transmission line 31a and an auxiliary transmission line 31b. In a normal operation state, the operating transmission line 31a is used for data transmission between the devices 36, and the auxiliary transmission line 31b stays in a waiting state. As shown in FIG. 1B, when a fault such as line break occurs at point A in the duplex transmission line 31 and the normal operation of the network is not maintained, two STs 30 adjacent to the fault position A form U-links (folded loops). Thus, the fault position A is separated from the network, and a data transmission network is reconstructed using the operating transmission line 31a and the auxiliary transmission line 31b.
In the conventional ring-type local area network, the auxiliary transmission line 31b of the duplex transmission line 31 is normally set in the waiting state, and is not used for data transmission. Thus, the entire network is not fully utilized for data transmission.
As seen from the above, there is an increased demand for an apparatus wherein a data transmission efficiency of a duplex transmission line in a ring-type local area network is fully utilized in a normal operating state, and, in case of a fault, the network can be reconstructed, like a conventional apparatus.